U.S. Pat. No. 3,475,169, issued to Lange on Oct. 28, 1969, discloses a process of electrophotographically screening color cathode-ray tubes by applying, over the image area, a conductive layer and a superimposed photoconductive layer preferably formed of a resin. A latent charge image is established on the photoconductive layer and then a developer is applied over the image area. The developer includes phosphor particles as well as a binder in a carrier liquid. Preferably, the binder is formed of the same resin as that included in the photoconductive layer and the resin encapsulates the phosphor particles. The application of the developer selectively deposits phosphor particles to develop the latent image and thereafter the excess developer is removed and the image dried. For a color screen, this same general process is carried out three times, once for each of the three color-emitting phosphors. The photoconductive layer tends to be hard and brittle so it is known to add a plasticizer, such as Piccolastic A-75 (a polymerized styrene homologue) or Piccoumaron 410-L (a terpene compound), each of which is a product of Pennsylvania Industrial Chemical Corp., to prevent cracking of the photoconductive layer and subsequent misregister of the phosphors. Alternatively, Plastolein 9066 LT (di-2-ethyl hexyl adipate), a product of Emery Industries, Inc. may be used. Using either of the two former-named materials, the ratio of plasticizer to resin (PVK, i.e., polyvinyl carbazole) is 1:1; whereas, for the latter-named material the plasticizer comprises about 13.3 weight percent of the resin. Plastolein 9066 LT is disclosed in U.S. Pat. No. 3,489,556 issued to Drozd on Jan. 13, 1970, and in U.S. Pat. No. 3,489,557 issued to Lange et al on Jan. 13, 1970.
U.S. Pat. No. 4,921,767, issued on May 1, 1990 to Datta et al., discloses a "dry" process for forming a CRT screen assembly. The "dry" process utilizes triboelectrical charged dry phosphor particles rather than phosphor particles suspended in a carrier liquid. The "dry" triboelectrically-charged phosphor develops the latent charge image formed on the photoconductive layer. The "dry" process requires fewer processing steps and is, therefore, more efficient than the Lange process. The initial steps in the "dry" process are similar to those described in the Lange patent in that a conductive layer and an overlying photoconductive layer are formed in the image area of the screen and a latent charge image is formed on the photoconductive layer. In the process disclosed in U.S. Pat. No. 4,921,767, op. Cit., the photoconductive layer does not contain a plasticizer because the aforementioned plasticizers (or their equivalents) are not capable of providing the critical electrical parameters of charge acceptance, light and dark decay rates, and light sensitivity without degradation, in the concentration of plasticizer necessary to prevent cracking of the PVK-based photoconductive layer.